Optical components, including fiber optic cables, connectors, transmitters, receivers, switches, routers and all other types of optical components have become the backbone of the modern telecommunication infrastructure. Due to their extremely low error rate and wide bandwidth, optical communication systems have supported an explosion in the growth of data communication systems, such as the Internet. With the Internet in its infancy, it is expected that the reliance on optical components and systems will only increase as the Internet becomes more closely intertwined with mainstream business and consumer applications.
Although the technology associated with optical communication systems and components has greatly advanced over the last decade and the use of such technology has accelerated, the technology associated with testing optical communication systems and components has greatly lagged.
Bit error rate (BER) measurements are a standard tool in verifying the performance of any digital optical communication system. Nevertheless, such tests remain an underutilized resource in understanding and diagnosing issues with such systems; particularly with respect to the receive-side optical front end. There are many contributing factors to this situation; chief among them are a lack of hardware and software resources, the time consuming nature of such measurements, and a lack of appreciation and understanding of the information content such measurements can provide.
The principle of BER measurements is simple: send digital data through a device and compare the digital result to the input data. The BER is given by the ratio of incorrectly identified bits to the total number of bits processed. In optical systems, BER tests are most commonly associated with determining the sensitivity of the optical receiver. Clearly, if the input optical power decreases enough, the receiver will begin generating errors. Receiver sensitivity is the input optical power required for a particular BER. Sensitivity is typically measured in dBm where:                               P          dBm                ≡                  10          ⁢                                           ⁢                      log            ⁡                          (                                                P                  ⁢                                                                           ⁢                  mW                                                  1                  ⁢                                                                           ⁢                  mW                                            )                                                          Eqn        .                                   ⁢                  (          1          )                    
Accordingly, 0 dBm corresponds to 1 mW. The result depends strongly on the measurement conditions including the quality of the transmitter, the amount of input optical noise, the BER required, the data rate and the data being transmitted. A typical measurement might involve a high quality transmitter, no added input noise, a well-defined pseudorandom bit sequence and a required BER of 1×10−10.
For a network designer, sensitivity is often regarded as the most important figure of merit for a receiver since it suggests a minimum input operating power for the device. A designer would ordinarily plan to operate the receiver with an input power high enough above the quoted sensitivity such that the expected error rate will not impact the reliability of the link. But how high above the sensitivity power level the receiver should be operated at is one of the fundamental questions that careful BER measurements can answer.
Due to the current state of technology for optical testing equipment, testing an optical component at many small increments of optical power over the full operating range is not realistic. To wit, via current practices, in order to perform these measurements, it is often necessary for a technician first to set the optical power of the test equipment to the desired optical power (which is an iterative process), and then must separately measure the number of errors at that optical power. Since the number of errors exhibited by optical equipment is extremely low, (i.e., 1×10−9 or less), it would necessitate a technician to continually attend to the testing equipment over a series of hours or days.
Known testing regimens avoid the problem of lengthy test procedures by requiring a technician to measure the BER over a few discrete levels of optical power. These results are then extrapolated throughout the entire operating range of the optical component to arrive at the behavior of the component over the entire operating range of the component. Extrapolating the results in such a manner increases the risk that the true behavior exhibited by the optical component at levels of power between the measured discrete levels will be missed. This can lead to later errors in the technical specifications for the particular component.
What is needed is a simple and effective system and method for efficiently testing the sensitivity of optical components.